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<br />Platania, Dudley, and Maruca. 2000. Drift of Fishes in the San Juan River 1991-1997. <br /> <br />FINAL <br /> <br />c) <br />,-, <br />~ <br />-1 <br /> <br />-.l <br />(....) <br /> <br />the variance-covariance matrix for hydrograph variables was examined and Pearson correlation <br />coefficients calculated among them. <br />Daily CPUE values were also compared to daily flow patterns. The mean daily discharge <br />within and between years was quite variable. Large rain events augmented flows briefly and many <br />were easily detected on the hydrograph. However, many smaller events only caused very minor <br />increases in flow but resulted in noticeable increases in stream turbidity and instream debris levels. <br />A rainstorm event is defined as resulting in noticeable increases in levels of turbidity and debris at UJe <br />sampling station even if measured increases in flow were negligible. <br /> <br />RESULTS <br /> <br />Summary of Data Collectian <br />A total of 17,525 individuals representing 12 fish species was collected in drift-nets over the <br />seven years of the study (Table 3) of which 13,683 (78.1 %) were drifting larval fish. Channel catfish <br />was the most abnndant of the drifting larval fish (n=4,384), followed by speckled dace (n~3,901), red <br />shiner (n=2,716), flannelmouth sucker (Calos/aml/s /aripinnis: n=I,448), bluehead sucker <br />(Catastamlls discoboll/s; n=954), fathead minnow (Pimepha/es prome/as; n=260), common carp <br />(Cyprinl/s calpia; n~15), and Colorado pikeminnow (n=5). A single roundtail chnb (Gila robl/sla) <br />was collected but its size indicated that it was an incidental capOlre. Fish species collected that were <br />not represented by drifting larvae (i.e., black bullhead [Ameil/"'s me/as], western mosquitofish <br />[Gambl/sia affinis], and largemouth bass [Micrapreros sa/maides]) comprised about 0.1 % of the total <br />catch. <br />The percentage of individuals that were drifting versus incidental varied between species, <br />sites, and years with the exception of Colorado pikeminnow (all individuals collected were drifting <br />larvae). The majority of red shiner taken during this study were classified as drifting fish (87%); the <br />proportion was the same at both Four Comers and Mexican Hat. This high percentage was largely <br />driven by large collections of red shiner in 1991 and 1995 when 98% of individuals were classified as <br />drifting. The percentage of red shiner that were drifting in other years ranged from 28% in 1992 to <br />81%in 1996. <br />Most common carp were classified as incidental capOlres (89%) and this pattern was <br />maintained annually at both sites with the exception of 1991 when the majority of individuals (56%) <br />were drifting. The percentage of drifting fathead minnow collected during the period of study (47%) <br />differed between sites (Mexican Hat-l 6% versns Four Comers-63%). This was primarily the result <br />of a large collection of non-drifting fathead minnow at Mexican Hat in 1993. Most speckled dace <br />collected were drifting (64%) with minimal variation between sites or years. Likewise, tile majority <br />of t1annelmouth sucker collected were classified as drifting (68%). However, the Four Corners site <br />had a relatively higher percentage of drifting flannelmouth sucker (89%) than did Mexican Hat <br />(46%). This difference was caused by a large collection of non-drifting t1annelmouth sucker <br />collected at Mexican Hat in 1993. Nearly all bluehead sucker (95%) and channcl catfish (98%) <br />collected were drifting individuals and this trend did not vary temporally or spatially. <br />Rank order of drifting fish varied by site (Figure 2) with speckled dace being the most <br />abnndant fish at Four Comers (38%) and channel catfish numerically dominating the catch at <br />Mexican Hat (54%). More drifting fish were collected at Four Comers (n~7,578) than at Mexican <br />Hat (n=6, 105). Red shiner, fathead minnow, speckled dace, t1annelmouth sucker, and blue head <br />sucker were all capturcd more frequently at Four Comers than at Mexican Hat while the converse <br />was true for channel catfish. <br />The number of drifting fish collected annually during this study ranged between 968 and <br />2.042 except in 1995 when 4,441 individuals were taken (Figure 3). Red shiner was the most <br />frequently collected tish in 1991, speckled dace nnmerically dominated two years (1993 and 1995), <br /> <br />13 <br />